The present invention relates to a receiver that is adaptable to a plurality of radio communication systems and more specifically to a receiver that collectively receives signals within the overall system band, selects a desired channel, and recovers original data transmitted on the selected channel.
In a communications environment in which there exist a plurality of radio communication systems that differ in frequency or band used, each radio terminal is required to have the ability to accommodate communications of the respective radio communication systems with a single receiver unit. From the following reasons it is difficult for conventional superheterodyne receivers to meet such a requirement with their practical arrangement.
In the superheterodyne receivers, a received signal from an antenna has an image-frequency signal eliminated therefrom and is then converted into a first intermediate-frequency signal. The first intermediate-frequency signal has an image-frequency signal eliminated again and is then converted into a second intermediate-frequency signal that is enough lower in frequency than the first intermediate-frequency signal. A signal on a desired channel is selected from the second intermediate-frequency signal and then demodulated. Where the received signal is an FM (frequency modulated), the second intermediate-frequency signal is subjected to amplitude limitation in an amplitude limiter and then applied to a frequency discriminator where the received signal is demodulated.
In the receiver thus arranged, an image suppression filter and a channel selection filter are used each of which consists of a passive filter such as a surface acoustic filter or a ceramic filter. The passive filter has great physical dimensions and moreover is costly, which prevents the downsizing and the cost reduction of the radio-frequency circuitry. In the passive filter, the central frequency and the bandwidth of the passband are usually fixed, and it is therefore difficult to make the central frequency and the bandwidth variable. In order for a radio terminal to receive simultaneously transmitted signals in multiple radio communication systems that differ in frequency and bandwidth, it is required to install a passive filter for each system. This cannot be granted from the viewpoint of any of physical dimensions and cost.
As a reception system that solves these problems, attention is paid to a direct conversion system. In a direct conversion receiver, a received signal from an antenna is fed into a quadrature demodulator, which, in two mixers, multiplies the received signal by two local signals that have substantially the same frequency as the received signal and are 90 degrees (.pi./2) out of phase with each other to thereby convert the received signal into two baseband signals having a 90-degree phase difference. The baseband signals are then applied to low-pass filters to select a signal on a desired channel.
FIG. 12 shows the manner in which a desired channel is selected. A signal on the desired channel is converted into zero frequency (DC) and then fed into a low-pass filter which eliminates other channels and interferential waves. The signal on the selected channel is amplified and then converted into a digital signal by an analog-to-digital converter. The digital signal is then applied to a digital signal processing section where original data is recovered.
In the direct conversion receiver, a received signal is directly converted into a baseband signal; thus, in principle there exists no image interference resulting from intermediate-frequency conversion. Accordingly, there is no need for an image suppression filter, consisting of a passive filter, that is essential to superheterodyne receivers. The channel selective low-pass filter can be implemented by an LSI. Thus, a broadband receiver can be implemented which does not require a separate device for each different radio communication system and is adaptable to a plurality of radio communication systems.
However, the direct conversion receiver requires to use inevitably the baseband demodulation as described above with resulting lack of flexibility. To overcome such a drawback, there have been proposals for signal processing systems which involve quadrature demodulation of a received signal and intermediate-frequency conversion of the demodulated signal, as described in Japanese Unexamined Patent Publications No. 59-196629 (FM RECEIVER) by way of example.
An attempt to arrange the receivers disclosed in the above publication so that they can accommodate a plurality of radio communication systems encounters the following problems. First, with the channel selective low-pass filter, it is required to make the passband variable over a wide range so that a selection can be made from channels including from a narrowband signal to a broadband signal over the overall system band. Further, in order for the filter to have the channel selecting function, it is required to use a very sharp high-order filter. On the other hand, the local oscillator in the quadrature demodulator is required to be capable of setting its oscillation frequency so as to accommodate all the radio communication systems that differ in channel spacing.
Thus, a severe burden is imposed on the performance of an analog circuit section in order to allow the arrangements as disclosed in the above-described publications to accommodate a plurality of radio communication systems. In general, in comparison with digital circuits, analog circuits have a large variability of characteristics and their characteristics are liable to vary with temperature and aging. To improve the performance and stability of receivers, therefore, it is not advisable to impose a burden on the performance of an analog circuit.